Techniques for nondestructive testing of typical damage in polymer-matrix composites are well developed, but the ability to monitor long-term degradation due to various kinds of environmental exposure is also important. In this paper, the ability of a capacitive sensor to detect the effects of thermal aging in a glass-fiber-reinforced bismaleimide composite is demonstrated. Samples were aged for 4 h at temperatures ranging from 548 to 648 K (275 to 375 °C). Real permittivity and the dissipation factor of the aged samples were first measured over the frequency range from 100 Hz to 2 MHz using a sandwich-type parallel plate capacitor arrangement. A reduction of approximately 30% in the real relative permittivity (from approximately 5.0 to 3.4) was observed for samples aged at 648 K (375 °C), compared with that for pristine samples, and an associated shift to higher frequency of relaxation in the dissipation factor was observed. Capacitance and dissipation factor were then measured on the same samples using a planar capacitive sensor with concentric interdigital electrodes, and qualitatively similar changes were observed. The planar capacitive sensor has potential for incorporation into a handheld probe or in-situ sensor for structural health monitoring.